Other Assets (Narrative) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) project	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) policy project	Dec. 31, 2018 USD ($) policy	Dec. 31, 2017 USD ($) policy	Dec. 31, 2014 USD ($)	Jan. 01, 2015 USD ($)
Business Acquisition [Line Items]
Number of policies owned | policy									9	9	9
Net income attributable to Brookline Bancorp, Inc.	$ 22,183	$ 22,596	$ 20,471	$ 22,467	$ 21,138	$ 22,460	$ 20,831	$ 18,633	$ 87,717	$ 83,062	$ 50,518
Affordable housing project
Business Acquisition [Line Items]
Number of investments | project	17								17
Maximum percentage of outstanding equity interest that can be invested by the entity in any single project									50.00%
Other non-interest income
Business Acquisition [Line Items]
Tax exempt BOLI income									$ 1,000	$ 1,000	$ 1,000
BankRI
Business Acquisition [Line Items]
Number of policies owned | policy									7	7
Net cash surrender value	$ 41,000				39,900				$ 41,000	$ 39,900
First Ipswich
Business Acquisition [Line Items]
Number of policies owned | policy									2	2
Net cash surrender value	$ 800				$ 800				$ 800	$ 800
ASU 2014-01
Business Acquisition [Line Items]
Net income attributable to Brookline Bancorp, Inc.												$ 500
Retained earnings													$ 1,100